Direct, non-destructive measurement of recess depth in a wafer

ABSTRACT

A direct and non-destructive method for measuring recess depth in a semiconductor wafer through use of a solvent, comprising:a) placing a recessed wafer into a track;b) pouring a solvent into the wafer;c) commencement of spinning the track-wafer-solvent to recess the solvent into the wafer trench solvent;d) subjecting the track-wafer-solvent from step c) to a subsequent spinning step to spin-off any remaining solvent on the surface of the wafer to leave the wafer trench filled with solvent;e) weighing the solvent-filled-trench wafer;f) subjecting the solvent-filled-trench wafer to heating to remove the solvent; andweighing the solvent-free wafer to determine the difference in weight, and using the density of the solvent together with the difference in weight to determine the recess depth.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an effective, simple, direct andnon-destructive method to measure recess depth in a wafer by putting thewafer into a track where a solvent is poured, commencement of spinningthe wafer in the track to fill-up the recess, and subsequentspinning-off of the remaining solvent so that no solvent remains on thewafer surface, weighing the wafer, heating to remove the solvent, andagain weighing to ascertain the difference in weight or the amount ofsolvent imbibed by the trenches together with solvent density, to permita simple calculation of the recess depth.

2. Description of the Prior Art

In the ASG and LOCOS resist or polysilicon recess process, control ofthe recess depth is crucial as the recess depth determines bothparasitic leakage and the capacity of the transistor, thus controllingthe storage time of the transistor.

At present, profiling techniques such as those performed with SEMs orAFM are used to monitor the recess depth within round or ellipticaldepths in a chip.

However, SEM measurements are time-consuming, costly and destructive. Onthe other hand, AFM measurements of recess depths in a trench is done inthe kerf, in which the trench size is large enough to accommodate thebulky AMF tip. Because the recess depth is closely related to the sizeof the trench, a correlation needs to be established between the largetrench on the kerf and the real trench in the arrays.

U.S. Pat. No. 6,275,297 discloses methods of and apparatus for measuringthe depth of structures on a semiconductor substrate. The measurement isaccomplished by a broadband light source that irradiates the recessedand non-recessed portions. A detector detects reflected light includinga first spectral component comprising light reflected from the recessedportions and a second spectral component comprising light reflected fromthe non-recessed portions, wherein at least one of the first and secondcomponents further comprises a third component comprising lightreflected from the dielectric layer. Spectral reflectance information ofthe detected rays is stored and a plot of reflectance intensity versuswavelength is generated. Depth geometries of the recesses and thedielectric layer are determined relative to the at least one referenceinterface based on an interferometric analysis of the plot.

An assembly for measuring a trench depth parameter in a work-piece isdisclosed in U.S. Pat. No. 5,691,540. The assembly comprises anultra-violet radiation source; a split fiber bundle having a firstbranch for propagating the ultra-violet radiation from the radiationsource to a lens, and a second branch; a lens for focusing the UVradiation to the workpiece and refocusing an ultra-violet interferencesignal to the second branch; and a detector responsive to theultra-violet interference signal received through the second branch. Thedetector transforms the ultra-violet interference signal to anelectrical signal which is a measure of a trench depth of the workpiece.The ultra-violet interference signal is developed when ultra-violetradiation propagates through the workpiece and reflects from its baseregion to thereby interfere with ultra-violet radiation that is directlyreflected by a workpiece surface which is different from the baseregion.

U.S. Pat. No. 6,124,141 discloses a non-destructive method for measuringthe depth at which the top surface of a buried interface is located in asemiconductor substrate. The method employs a Fourier Transform Infraredmeasurement, and comprises subjecting the semiconductor substratecontaining the buried interface to a beam of infrared light and thendetecting and analyzing the spectrum of a return signal by a Fourieranalysis. The spectrum as analyzed by the Fourier analysis is thencompared to calibration spectra to thereby determine the depth of thetop surface of the buried interface. The invention also uses a devicefor determining the depth of a buried interface below the surface of asemiconductor substrate. That device comprises a FTIR spectrophotometerwhich illuminates the substrate with a source of infrared radiation andwhich produces a Fourier transform of a return signal reflected from thesubstrate. The device includes a library of stored calibration spectra,along with means for comparing the Fourier transform return signal tothe calibration spectra to determine the depth of the buried interface.

U.S. Pat. No. 4,840,487 discloses an apparatus for measuring etchingpits by employing a light source having a small absorptivity withrespect to a groove or pit as an object of irradiation to insure asatisfactory change in the interference intensity of the detracted lightwhich is reflected from the object. The apparatus includes a detectormeans provided with the freedom of movement in two axial directionswhich are perpendicular to each other and in the direction of rotation.In addition, as a laser source, a He—Cd, N₂ or Ar laser may be employedin addition to a He—Ne laser to detect changes in their interferenceintensities, and the etch depth is calculated on the basis of thedetected changes.

There is a need in the art of measuring recess depth in the wafer for aneffective, simple, direct and non-destructive measurement technique forrecess process control, especially with device dimensions of groundruleshrinkage to 0.13 microns and below, as the current profiling techniquesare becoming increasingly more costly and difficult to correlate.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an effective andnon-destructive measurement technique for recess process control ordepth measurement in a semiconductor chip, where the device dimensionsor ground rules have shrunk to 0.13 microns and below.

Another object of the present invention is to provide a simple,non-destructive measurement technique for recess process control ordepth where the device dimension or groundrules have shrunk to 0.13microns and below without employing SEM or AMF profiling techniques.

A further object of the present invention is to provide a direct,non-destructive measurement technique for recess process control ordepth measurement in a semiconductor, where device dimension or groundrules have shrunk to 0.13 microns and below, that are less costly andless difficult to correlate.

In general, the effective, simple, direct and non-destructive method formeasuring recess depth in a semiconductor chip of the invention isthrough the use of a solvent, and entails:

placing the recessed wafer into a track where a solvent is poured ontothe wafer and commencing spinning of the wafer to recess the solventinto the trench and fill it up;

subsequently spinning off the remaining solvent so that no solventremains on the surface of the wafer;

weighing the wafer;

heating to remove the solvent;

again weighing to ascertain the difference in the two weights or theamount of the solvent imbibed by the trenches; and

calculating recess depth premised around the density of the solvent andthe weight difference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a Scanning Electron Micrograph of a typical wafer profileafter resist recess and etching.

FIGS. 2a-2 e (collectively FIG. 2) are schematics of the non-destructivemethod of measuring recess depth of a wafer utilizing a solvent inaccordance with the invention process.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION

Reference is now made to FIG. 1 which shows scanning electronmicrographs (SEMs) of a typical wafer profile after resist recess andetching. The recess material Accuflow 913 EL from Honeywell. The PORresist recess etch recipe was applied to the wafer. From x-sectionpictures of the array center, and array edge, the most important data iscollected; namely, the recess depth. In addition to the recess depth,information regarding the maximum variation is also collected, that is,the maximum variation within the wafer is 0.22 microns, within the arraycenters 0.11 micron and from the array center to the edge 0.14 micronsor smaller.

For a 110 nm DRAM product, there is a 308 chips per 8 inches of wafer,and a half billion trenches per chip. Assuming that each trench has awidth of 1.25 nm, a length of 220 nm, and a depth of 1.3 microns, thetotal volume of trench that would have potential to be filled by asolvent is approximately 4.3 mm³. The particular solvent utilized has adensity or specific gravity of about 1.4 g/cm³, and the weightdifference is about 6 mg. The balance scale utilized is easily capableof weighing up to 100 g with a resolution of up to 0.05 mg. Thiscorresponds to less than 1% noise level; in other words, easymeasurement of a trench with recess depth of 1.3 microns can be madewith an accuracy of up to 13 nm.

FIG. 2 is a schematic of the invention process for measuring the recessdepth of a semiconductor chip. After recess, the wafers are put into atrack where several mil of a solvent with a density of 1.4 g/cm3 ispoured into the wafers, whereupon spinning is started. Because of thecapillary force between the surface of the trenches and the solvent, thesolvent easily recesses into the trench and fills it up. A subsequentspinning step is used to spin off the remaining solvent and render awafer with trench filled with the solvent. However, at this juncture nosolvent remains on the surface of the wafer. The wafer is weighted by abalance and then placed into a hot plate to remove the solvent byevaporation, whereupon the wafer is weighed again. The difference of theweight is the weight of the solvent imbibed by the trenches.

FIGS. 2a to 2 b depicts a wafer after it is put into a track and solventis first applied, followed by commencement of spinning.

FIG. 2c depicts the wafer filled with the solvent.

FIG. 2d depicts the wafer after the second spinning process.

FIG. 2e depicts the wafer after it is put into a balance and weightedfollowing removal of the solvent by heating to obtain weight W2.

The following calculations will show the amount the solvent that can beimbibed by 8 inches of wafer in the invention process:

The volume of one DT trench is

1.3 um*Pi*125/2 nm*220/2 nm=2.8 10⁻¹¹ mm³

The number of DT per wafer is 308×0.5×10⁹=1.54×10¹¹

The total volume of DT trenches per wafer is 2.8×10⁻¹¹ mm³×1.54×10¹¹=4.3mm³

Assume the density of solvent is 1.4 g/cm³, the weight of the solventthat can be imbibed by trenches=4.3 mm³×1.4 g/cm³=6.0 mg

The balance used can easily measure a 100 g object with resolution up to0.05 mg.

The invention method takes advantage of capillary force between thesurface of the trenches and the chosen solvent. Capillary force is thereaction between contacting surfaces of a liquid and a solid thatdistorts the liquid surface from a planar shape; it works veryeffectively in the size range of 1-100 microns.

The invention method is not limited to 110 nm DRAM products or recessprocesses. The method is applicable to any process wherein there is aneed to monitor the depth of huge numbers of trenches with relativelyuniform depth across the wafers.

We claim:
 1. A method for measuring recess depth in a semiconductorwafer through use of a solvent, comprising: a) placing a recessed waferinto a track; b) pouring a solvent into the wafer; c) commencement ofspinning the track-wafer-solvent to recess said solvent into the wafertrench solvent; d) subjecting the track-wafer-solvent from step c) to asubsequent spinning step to spin-off any remaining solvent on thesurface of said wafer to leave the wafer trench filled with solvent; e)weighing the solvent-filled-trench wafer; f) subjecting thesolvent-filled-trench wafer to heating to remove said solvent; and g)weighing the solvent-free water to determine the difference in weight,and using the density of the solvent together with the difference inweight to determine the recess depth.
 2. The method of claim 1 whereinsaid solvent is an organic solvent.
 3. The method of claim 2 whereinsaid solvent is characterized by a density of about 1.4 g/cm³.
 4. Themethod of claim 3 wherein said semiconductor wafer has a devicedimension of 0.13 μm or less.
 5. The method of claim 1 wherein saidsemiconductor wafer includes 308 chips per 8 inches of a wafer, and ahalf billion trenches per chip, each chip being a 110 nm DRAM product.6. The method of claim 5 wherein each trench has a width of 125 nm, alength of 220 nm and a depth of 1.3 μm.
 7. The method of claim 6 whereinthe total volume of trench filled-up with said solvent is about 4.3 mm³.8. The method of claim 7 wherein said weight difference is about 6 mg.9. The method of claim 1 wherein said recess is a polysilicon recess.10. The method of claim 9 wherein said polysilicon recess results froman ASG or a LOCOS process.
 11. A method of measuring recess depth in asemiconductor wafer, the method comprising: providing a wafer includinga plurality of recesses, the wafer having a device dimension of 0.13 μmor less; filling each of the recesses with a liquid; determining aweight of the wafer with the liquid in each of the recesses; determininga weight of the wafer with no liquid in any of the recesses; anddetermining in a recess depth for each of the recesses based upon adifference in the weight with the liquid and the weight with no liquid.12. The method of claim 11 wherein the wafer is placed in a track priorto filling each of the recesses with a liquid.
 13. The method of claim11 wherein filling each of the recesses with a liquid comprises pouringa liquid onto the wafer and subsequently spinning the wafer.
 14. Themethod of claim 13 wherein spinning the wafer comprises performing aninitial spinning step to cause the liquid to enter each of the recessesfollowed by a subsequent spinning step to remove remaining solvent froma surface of the wafer.
 15. The method of claim 11 and furthercomprising heating the wafer to remove liquid from the recesses.
 16. Themethod of claim 15 wherein the heating step is performed beforedetermining a weight of the wafer with no liquid in any of the recesses.17. The method of claim 11 wherein the liquid comprises an organicsolvent.
 18. The method of claim 11 wherein the liquid is characterizedby a density of about 1.4 g/cm³.
 19. The method of claim 11 wherein saidsemiconductor wafer has a device dimension of 0.13 μm or less.
 20. Themethod of claim 11 wherein said semiconductor wafer comprises a DRAMproduct.